Data usage

After signing a 24 month contract with Cell C for a router, on 28 August 2015. I checked my data usage on the 12 Sept. It reflected that I've used 7 gigs already. I went to the Cell C store at the Garden Route Mall & she informed that on the 12 Sept I used 4gigs. I asked her if she can tell me on what did I use the 4g, cause the router was turned off since late afternoon, she informed she can't see. She contacted the call centre but they are also unable to assist me. When I took out the contract i asked if a limit can be set? The consultant told me no limit can be set. Today a different consultant at the same shop phones the help line & a limit is set. I then wanted to cancel the contract & was informed I will have to pay R861,56. I asked her that in the agreement I signed, it made no mention of a extra fee I must pay if I want to cancel a contract. 17 days in my contract and I want to cancel. Answers cannot be given to me when I asked. The consultant that I signed with **** to me when he said no limit can be set & if no mention is made of a cancellation fee in the contract, why must I pay the R861,56?